For many years, processes have been available to facilitate the deposition of electrodeposited metals onto plastic substrates. Typically, the process involves the steps of:                1) Etching the plastic in a suitable etching solution such that the surface of the plastic becomes roughened and wetted so that the subsequently applied deposit has good adhesion;        2) Activating the surface of the plastic using a colloidal or ionic solution of a metal (usually palladium) capable of initiating the deposition of an autocatalytically applied metal coating (e.g., copper or nickel);        3) Depositing a thin layer of autocatalytically applied metal; and        4) Carrying out electrodeposition of metal on the metallized plastic substrate.Typically, layers of copper, nickel and/or chromium will be applied to produce the finished article.        
The most widely used plastic substrates are acrylonitrile/butadiene/styrene copolymers (ABS) or ABS blended with polycarbonate (ABS/PC). These materials are readily formed into components by the process of injection molding. ABS comprises a relatively hard matrix of acrylonitrile/styrene copolymer and the butadiene polymerizes to form a separate phase. It is this softer phase of polybutadiene (which contains double bonds in the polymer backbone) which may be readily etched using various techniques.
Traditionally, the etching has been carried out using a mixture of chromic and sulfuric acids operated at elevated temperature. The chromic acid is capable of dissolving the polybutadiene phase of the ABS by oxidation of the double bonds in the backbone of the polybutadiene polymer, which has proven to be reliable and effective over a wide range of ABS and ABS/PC plastics. However, the use of chromic acid has become increasingly regulated because of its toxicity and carcinogenic nature. For this reason, there has been considerable research into other means of etching ABS plastics and a number of approaches have been suggested to achieve this.
For example, acidic permanganate is capable of oxidizing the double bonds in the polybutadiene. Chain scission can then be achieved by further oxidation with periodate ions. Ozone is also capable of oxidizing polybutadiene. However, ozone is extremely dangerous to use and highly toxic. Likewise, sulfur trioxide can be used to etch ABS, but this has not been successfully achieved on a typical plating line. Other examples of techniques for etching ABS plastics are described in U.S. Pat. Pub. No. 2005/0199587 to Bengston, U.S. Pat. Pub. No. 2009/0092757 to Sakou et al., and U.S. Pat. No. 5,160,600 to Gordhanbai et al., the subject matter of each of which is herein incorporated by reference in its entirety.
More recently, it has been discovered that ABS and ABS/PC plastic can be etched in a solution containing manganese(III) ions in strong sulfuric acid as described in U.S. Pat. Pub. No. 2013/0186774 to Pearson et al., the subject matter of which is herein incorporated by reference in its entirety.
In order to plate plastic components, they are attached to plating racks which transmit the electrical current to the sensitized and metallized plastic components. After assembly of the plating racks but prior to use, it is desirable to cover at least a portion of the rack with an insulating coating of plastic or the like and a preferred and commonly used insulating coating is a plastisol such as a polyvinyl chloride resin dispersed in a plasticizer (i.e., a “PVC plastisol”). The use of a plastisol coating prevents the rack from being covered with metal during the electroplating process. The use of plastisols, such as PVC plastisols, for rack plating is well known as described for example in U.S. Pat. No. 3,357,913 to Zavarella and U.S. Pat. No. 4,297,197 to Salman, the subject matter of each of which is herein incorporated by reference in its entirety.
The use of chromic acid in the etching stage prior to activation is effective in modifying the surface of the plastisol coating so that it is resistant to metallization after being coated with a palladium activator (usually a colloid of palladium and tin). However, when chromic acid is replaced with other etching techniques, for example, using processes containing permanganate or manganese (III), the plastisol coating of the plating rack becomes coated with the activator and subsequently becomes coated with a layer of either nickel or copper in the electroless plating stage.
Thus, a major problem with all of the currently known methods that do not utilize chromic acid in the etching stage is that rack coatings tend to become plated in the subsequently electroless plating stage. This phenomenon is known as “rack plate up” and has been a major problem with any form of chrome-free etching technology.
There is a need in the art for a modified PVC plastisol coating that is capable of being used in a chrome-free etch process without subsequent metallization of the rack and that does not contain ingredients that leach out of the plastisol and cause deleterious effects in the treatment tanks.